Variation in the PaO2/FiO2 ratio with FiO2: mathematical and experimental description, and clinical relevance

Table 1 Numbers of patients changing disease group with increasing inspired oxygen fraction (FiO₂) across the relevant range

Low FiO₂	High FiO₂
	Normal	Mild hypoxemia	Acute lung injury	Acute respiratory distress syndrome
'Effective shunt' model	n = 23	n = 15	n = 40	n = 38
Normal (n = 64)	23	14	27	0
Mild hypoxemia (n = 20)	0	1	13	6
Acute lung injury (n = 14)	0	0	0	14
Acute respiratory distress syndrome (n = 18)	0	0	0	18
Ventilation/perfusion and shunt model	n = 42	n = 19	n = 31	n = 24
Normal (n = 56)	39	12	5	0
Mild hypoxemia (n = 19)	3	6	9	1
Acute lung injury (n = 23)	0	1	16	6
Acute respiratory distress syndrome (n = 18)	0	0	1	17

Patients classified into disease groups at the lowest and highest FiO₂ level in the range, according to the following partial pressure of oxygen in arterial blood (PaO₂)/FiO₂ ratio criteria: normal (PaO₂/FiO₂ > 47 kPa) [19], mild hypoxemia (40 kPa ≤ PaO₂/FiO₂ < 47 kPa), acute lung injury (27 kPa ≤ PaO₂/FiO₂ < 40 kPa) [4,5], and acute respiratory distress syndrome (PaO₂/FiO₂ < 27 kPa) [4,5].

ISSN: 1364-8535